1. Field of the Invention
The invention relates to the use of modified water-soluble polymers as crosslinking assistants, optionally in combination with film-forming polymers, epoxy components or other hardeners, for example for coatings or adhesives, such as fiber-coating compositions, paper-coating compositions or chemical products for use in civil engineering.
2. Description of the Related Art
Water-soluble polymers, for example polyvinyl alcohols, are widely used, inter alia as additives for coatings or adhesives and particularly as a protective colloid for polymers based on ethylenically unsaturated monomers. However, the high water solubility of such polymers often becomes adversely conspicuous when in-service products become exposed to water. To improve waterfastness, compositions comprising water-soluble polymers have for example been employed in combination with hardeners or crosslinkers. Crosslinkers having isocyanate or aldehyde groups have thus been used to crosslink polyvinyl alcohol at room temperature. After crosslinking, the products do have an improved waterfastness, yet crosslinkers of this type are a health concern which necessitates higher safety measures for their use. In addition, the crosslinking reaction usually does not go to completion, so the in-service products may evolve harmful compounds for a prolonged period.
Known crosslinking systems further include epoxy-containing compounds which, however, only lead to crosslinking at comparatively high temperatures, for example above 100° C. But there are many applications, for example in civil engineering, such as tile adhesives or leveling screeds, where the crosslinking reaction has to take place at room temperature. Amines may additionally be added to crosslink epoxy-containing compounds at room temperature. With this procedure, the crosslinking of the water-soluble polymers is insufficient and the wetfastness of products obtained therefrom is still not fully satisfactory.
Epoxy resins are extensively recommended as crosslinkers, including in particular for protective colloid-stabilized polymer powders, as for example described in WO 2012/177448, US 2012/0329909, US 2012/0329908, US 2012/0329907 or WO 2010/086217. EP 0721004 describes water-redispersible polymer powders comprising film-forming functionalized polymers and also reactive compounds, wherein said reactive compounds are suitable for crosslinking on the functionalized polymers. US 2001/0024644 describes protective colloid-stabilized polymer powders whose base polymers contain epoxy monomer units. EP 0896029 teaches crosslinkable polymer compositions based on water-insoluble polymers as well as water-soluble polymers, wherein at least one of the two polymers contains crosslinkable groups capable of forming covalent bonds with insoluble organic or inorganic solids.
EP 0885246 discloses the use of water-soluble polymers based on 2-acrylamido-2-methylpropansulfonic acid (AMPS) and N-methylolacrylamide solution (NMA) as protective colloids for water-insoluble polymers. Protective colloids of this type, however, tend to evolve formaldehyde, and this is deprecated for health reasons. Moreover, protective colloids based on such monomers are not resistant to water. EP 0723975 describes the copolymerization of ethylenically unsaturated epoxides and also of further ethylenically unsaturated monomers in the presence of small amounts of protective colloids and also the employment of such products in coatings or binders to improve the adhesive properties thereof. DE-A 102012223620 finally teaches protective colloids for film-forming polymers that are obtainable by polymerization of optionally functionalized ethylenically unsaturated monomers in the presence of water-soluble polymers.
A further issue is the inadequate staining resistance of the abovementioned coatings, for example on contact with colored beverages, for example fruit juices or coffee.